Untraceable in a legal/administrative sense, it could still be traced using forensics.

In the US the lower receiver is the component that is legally considered a firearm. They're required to have serial numbers and are subject to the same laws as a fully functional gun. The other components are just hunks of metal in a legal sense and may be sold and manufactured without restriction. If you are able to fabricate a lower receiver by yourself you can buy the rest of the components you need to assemble a fully functional rifle without any records of its existence or creation.

According to the BATF a block of metal doesn't become a lower receiver until it is more than 80% complete. This led to the creation of a market "80% lowers" that can be legally sold without restriction and completed by the buyer. The hardest milling is done by the manufacturer and they are designed to be completed by inexperienced users with common tools. There's even a company that will sell you a CNC mill preprogrammed to finish milling the receiver.

Probably just in terms of serial number / manufacturer stamp / etc which are what live on the lower.

Well if you're planning to trace weapons by their serial number then you're already going to be defeated by a simple screwdriver, let alone 3D printing.

You'd be surprised how well a serial number can be recovered even after it's been filed away. The stamping process that imprints each digit affects the metal underneath as well. Even when the physical characters have been filed away, the serial is readable using some advanced techniques[1]

[1] https://www.washingtonpost.com/national/health-science/scien...

Bureaucracy is a bigger issue. All guns registered in the US are required by law to have their serial numbers recorded only on paper, not on computers. This means that even if you know the serial number of a gun used in a crime, it can take weeks or months to trace it to a purchaser. The reverse search (find all serial numbers associated with a particular person) is basically impossible.

Source: https://www.gq.com/story/inside-federal-bureau-of-way-too-ma...

Barrels, in the US at least, are trivial to replace.